1. Field of the Invention
This invention relates to communication systems and, more particularly, to a communication system and method for synchronizing a plurality of network nodes after a network lock condition occurs.
2. Description of the Related Art
The following descriptions and examples are given as background only.
A communication system is generally well known as containing at least two nodes interconnected by a transmission line. Each transmission line can accommodate not only digital data, but also data that can arrive as voice data, audio data, video data, or bursts of data derived from a computer domain. An optimal transmission line is, therefore, one that can receive information from a multimedia device, herein defined as any hardware and/or software module that can transfer information in whatever form upon the network. The transmission line can either be a copper wire, an optical fiber, or a wireless transmission medium.
There are many types of multimedia devices. For example, a multimedia device can include a telephone, a compact disc (CD) player, a digital video disc (DVD) player, a computer, an amplifier, a speaker, or any device that can send and receive different types of data across the transmission line of the network.
Popular types of data sent or received by multimedia devices include streaming data and packetized data. Streaming data is data that has a temporal relationship between samples produced from a source port onto the network. That relationship must be maintained in order to prevent perceptible errors, such as gaps or altered frequencies at the destination port. Packetized data need not maintain the sample rate or temporal relationship of that data and, instead, can be sent as disjointed bursts across the transmission line.
Streaming data can be sent either synchronously or isochronously across the network depending on the frequency difference between the local clock of the source port (or destination port) and the network frame transfer rate, or the rate at which data frames are transferred across the transmission line. If the local sample rate (sometimes denoted “fs”) of the node is the same frequency as the network frame transfer rate (or frame synchronization rate, “FSR”) of the transmission line, then the streaming data can be sent synchronously across the network. In many instances, FSR may be dissimilar from the local sample rate, fs, of a multimedia device located within a node. Thus, the local sample rate must be changed (or converted), or the streaming data must be sent isochronously across the network, where isochronous transfer protocols are used to accommodate the frequency differences in order to prevent perceptible gaps, errors, jitter, or echo.
Regardless of how data is being sent across a transmission line, the data must nonetheless be referenced to a clock. The clock (sometimes known as a master clock) placed in one node (sometimes known as a master node) synchronizes the transmission from that node across the transmission line. The remaining nodes of the network (sometimes known as slave nodes) attempt to synchronize their local clocks to the master clock signal, typically by utilizing some form of clock recovery circuit (e.g., a phase locked loop, “PLL”, digital signal processor, “DSP,” or phase comparator) to lock onto the frequency of the master clock signal. A network lock condition occurs once all nodes of the network have locked on to the master clock signal. The process of locking or synchronizing a local clock signal to a master clock signal is typically performed after power-up, reset or a loss of lock (i.e., an unlock condition) on the network.
Once a network lock condition occurs, the data received by the slave nodes will have the correct frequency but will be phase shifted with respect to the data transmitted by the master node. This phase shift is due to delays attributed to each active node and additional accumulated delays due to tolerances in phase lock within the active nodes. The delays from each active node can vary within some range every time a power-up, reset or unlock to lock event occurs. These delays can also vary between different nodes on the network. Therefore, each node on the network may attribute a different fixed, but unpredictable amount of delay each time a network lock condition occurs. Such unpredictable delays are undesirable in many network applications.
A need exists for a communication system and method for synchronizing a plurality of network nodes after a network lock condition occurs. In particular, a communication system and method is needed to compensate for the unpredictable phase delays, which are produced at the network nodes each time a network lock condition occurs. Such a need is met herein by simultaneously generating a synchronizing trigger signal at one or more nodes of the network.